Watercraft are often powered by an outboard motor positioned at a stem of the craft The outboard motor has a powerhead and a water propulsion device, such as a propeller. The powerhead includes a cowling in which is positioned an internal combustion engine, the engine having an output shaft arranged to drive the water propulsion device.
Generally, the motor is connected to the watercraft in a manner which permits the motor to be "trimmed" up and down. For example, the motor may be connected through a horizontally extending pivot pin to a clamping bracket which attaches to the watercraft. In this manner, the motor may be moved in a vertical plane about the axis of the pin. This allows an operator of the watercraft to raise the propeller out of the water of place it deep in the water dependent upon the trim angle of the motor.
In addition, the motor is arranged to turn left and right about a generally vertically extending axis. This arrangement permits the operator of the watercraft to change the propulsion direction of the motor, and thus change the direction in which the watercraft is propelled.
The size of the motor, especially the powerhead portion which includes the motor, effects the air drag associated with the watercraft. It is desirable for the motor to have a small profile to reduce the air drag. In addition, it is generally desirable for the engine to be compact, since this makes the task of trimming and turning the motor less difficult
The engine typically is of the internal combustion type with one or a plurality of cylinders. In this type of engine it is desirable that all of the cylinders receive a uniform mixture of air and fuel.
Typically a charge forming device such as a carburetor introduces fuel into air passing through an intake passage. The fuel and air mixture is then routed through individual intake passages leading to each cylinder of the engine. If unequal amounts of fuel are delivered to the cylinders, many problems may result. For example, the engine may not operate smoothly, one cylinder may produce high emissions, and one cylinder may run much hotter than the other making even cooling of the engine difficult.
An example of an intake manifold of the prior art which suffers from unequal fuel distribution is illustrated in FIGS. 10(a) and 10(b). As illustrated therein, a manifold M includes a main passage P extending from a mounting flange F1. The flange F1 may be connected to a carburetor which introduces fuel into the air passing therethrough. The manifold M branches into a first branch B1 and second branch B2, each branch corresponding to a cylinder of an engine. Each branch B1 ,B2 defines a passage therethrough leading from the main passage P.
As illustrated, the intake for one cylinder is much lower than the other, the second branch B2 slopes downwardly from an axis along which the main passage P and first branch B1 are located. The manifold M is connected to the engine at a second flange F2.
In this arrangement, fuel tends to flow in a larger volume through the branch B2, as the air and fuel mixture which reaches a divider D at the end of the main passage P tends to flow with the air of gravity to this branch B2 defining the lowest point of the manifold M.
In some arrangements, the first branch B1 extends upwardly from the main passage P to the flange F2. In that arrangement, fuel actually drains back in the direction of the second passage B2 further compounding the problem of unequal fuel distribution.
Attempts to solve this problem are compounded by the fact that the intake manifold must not be excessively large or the size of the engine is undesirably increased. Thus, an improved intake manifold which provides for equal fuel distribution, but, without requiring long branch pipes is desired.